The present invention relates to a dry reversal developer for electrostatic photography comprising a carrier having ferromagnetic properties and a toner and an electrostatic photographic method for reversal development using a magnetic brush and using the abovementioned developer.
A magnetic brush method has been well known as an electrostatic photographic method using a dry reversal developer comprising carrier and toner. The magnetic brush method and the developer therefor have been disclosed, for example, in the U.S. Pat. No. 2,874,063 specification and the Japanese Patent Publications 40-10866 (published June 1, 1965) and 48-8139 (published Mar. 12, 1972). In those methods, a carrier having ferromagnetic properties is usually an iron particle carrier, to which toner having an average particle size of 1-50 .mu.m is added as an essential ingredient to constitute the powdery developer, which is made brush-like by a magnetic force of the carrier and deposited in a pattern of an electrostatic latent image to develop the image which is then transferred and fixed to an image recording paper. In these methods, the developer having a volume specific resistance of not higher than 1.times.10.sup.8 .OMEGA.-cm (the measuring method for which will be discussed later) has been used, which produces a proximate counter electrode effect resulting in the elimination of edge effect and a record having an excellent graduation. Accordingly, a high quality image can be produced by using a so-called normal development in which an image appears on those areas on which charges exist. In the meantime, as an approach for a recent trend of high speed and high density recording, a laser beam nonimpact printer by electrophotographic method in which an image is optically written by a laser beam and the image is reverse-developed, that is, the exposed area is developed, has been put into practice. When the prior art developer is incorporated in the reversal development process using the magnetic brush, however, it has been found that satisfactory development is not attained by the proximate counterelectrode effect of the magnetic brush. The quality of development is poor, particularly when a photoconductor having a high residual potential is used or in a high speed recording in which sufficient amount of exposure or developing time is not available. Thus, an approach in which the entire developing apparatus is insulated from ground and a bias voltage is externally applied thereto or an approach in which the bias voltage is not applied but the entire developing apparatus is insulated from the ground or a resistor of high resistance or a capacitor is connected in series with the developing apparatus to apply a self-biasing voltage has been employed. In those approaches, however, when the photoconductor includes a defect such as a pin-hole, loss of character occurs over an area broader than the area of the defect such as a pin-hole due to the shorting of the bias voltage, or the defect area expands by the discharge upon shorting. Furthermore, the insulation of the entire developing apparatus is troublesome and the bias voltage may be shorted by the accumulation of scattered carriers. Although an approach to use an insulative sleeve instead of insulating the developing apparatus has been proposed, it does not always resolve all of the above problems but creates additional problems in the durability of the sleeve and the manufacture of the sleeve. Accordingly, the development of a reversal development process, including use of a developer having different characteristics than that required in the normal development, has been sought.